The present invention generally relates to balloon catheters for implanting stents within a body lumen. More particularly, the invention pertains to improvements to such catheters in order to more effectively and reliably achieve a uniform expansion of such stents while minimizing trauma to the vessel wall.
Stents or expandable grafts are implanted in a variety of body lumens in order to prevent collapse and thereby maintain the patency of such lumens. In the case of angioplasty applications, stents may also be implanted to prevent restenosis and thereby similarly maintain patency in the affected blood vessel. The stent is introduced into the body in a collapsed state to facilitate its transport to the deployment site where it is subsequently expanded. One approach for achieving expansion requires the stent in its contracted state to be fitted about an inflatable balloon disposed near the distal end of a catheter. The entire assembly is advanced through the vasculature and maneuvered into the desired position adjacent the section of lumen in need of support. Once in position, the balloon is inflated causing the stent to expand and engage the lumen walls. Various stent configurations and mechanisms have been devised to lock the stent into its expanded state in order to provide the requisite radial support to the lumen. Once the stent is fully expanded, the balloon is deflated and the catheter removed to leave the stent in place. Some stents are designed to permanently remain implanted while others are formed of materials that eventually become absorbed by the body.
The effectiveness of a stent can be diminished if it is not uniformly implanted within the body lumen. Stents expanded by the inflation of a balloon have a tendency to undergo a disproportionate rate and amount of radial expansion at their proximal and distal ends due to the typical drop off in hoop strength encountered near the ends of the structure. Thus the balloon expands along the path of least resistance in a "dog bone" pattern which is similarly imparted to the stent. Such non-uniformity in the implanted stent may be problematic in that the desired flow diameter of the stent may not be achievable without forcing the stent ends deep into the lumen tissue. In the case of arterial applications, the non-uniformity of surfaces encountered by blood flow may cause turbulence which in turn may lead to thrombosis.
A further disadvantage inherent in many stent configurations currently in use is that the structure undergoes longitudinal contraction as it is expanded radially. This characteristic, in conjunction with the tendency of the stent ends to expand first, has the potential for inflicting trauma on the lumen in which the stent is being deployed. Because the initial expansion of the stent ends may cause such ends to project into the lumen tissue, the subsequent radial expansion and hence longitudinal contraction of the center section would cause such ends to be pulled across the tissue. The rubbing or scraping of the stent against the tissue could cause injury.
This problem has been previously addressed in a number of ways including for example, the use of shape defining sleeves that are fitted about the balloon. It is the intent of such system to match the radial force profile generated by the balloon to the hoop strength of the stent and thereby achieve a constant rate of expansion over the length of the stent.
Alternatively, multiple balloon systems have been employed in an effort to control the expansion of the stent. In one system, "control" balloons are positioned proximally and distally to a centrally disposed expansion balloon. The two control balloons check axial growth of the expansion balloon and hence prevent axially displaced lateral loads to be placed on the stent. As a further alternative, the stent is positioned over multiple balloons of varied compliance arranged in series along the catheter. By sequencing the inflation of the balloons such that the central balloon is inflated first, a more uniform implantation of the stent is achieved.
Nonetheless, those concerned with the design, development and use of stent implantation systems recognize the desirability of further improvements in terms of performance efficiency, reliability and reductions in the cost of manufacture.